new GoGame property reasonForComputerIsThinking

we need to be able to distinguish between different reasons why the computer
is busy thinking (e.g. so that we can update the status view).

the old computerThinks property is now merely used as a convenience
property that simplifies querying the new property in those cases where a
simple boolean state is required.
status view now correctly displays "updating player influence" (#18)
do not display continue button while player influence is being updated (#18)
enable territory statistics on launch (#18)
NSCoding archiving/unarchiving is faster due to reduced archive size

the NSCoding archive consists for the main part of two types of archived
objects: GoPoint and GoBoardRegion objects. the archive size can be
reduced by not writing all properties to the archive.

the optimization works like this:
- determine a number of properties in GoPoint and GoBoardRegion that
  are anticipated to have the same value for most of the time of the app's
  lifecycle
- when the time comes to write an archive, check if a property has the
  anticipated value: if yes do NOT write the property value to the archive.
  as a result, the archive size shrinks in most - but not all - situations
- when the time comes to read an archive, check if an encoded value
  exists in the archive: if not, simply assign the anticipated value to the
  property

two results of this optimization:
- for a 19x19 board that is almost empty, the archive size is reduced by
  roughly 40% of the original size (70kb instead of 116kb)
- for a 19x19 board that is almost full, the archive size is reduced by
  roughly 20% (133kb instead of 167kb)

the expected effects of the optimization are
- reading/writing the archive from/to disk should be faster due to the
  reduced size of the archive file
- decoding the archive should be faster because there is less data to
  decode
- encoding the archive should be faster because there is less data to
  encode